My experience with these in the CJ Premier 12 monoblocks found nice results from the RCA clear top 6FQ7 and the 1958 Sylvania 6CG7. No experience with ModWright.
I'd watch for black plate 6FQ7 Raytheons. Usually mid 50's vintage. Not overly common, but also not that expensive to get the best of that type of tube, so search away! The black plate Raytheons are a step ahead of all the others in my experience, a more relaxed presentation and a deeper soundstage with excellent image placement.
Tung Sol 6CG7 black plates are very good as well, about the equal of the Raytheons.
The 6CG7 is basically a 6FQ7 without the electrostatic shield between the plates, although some are mis-marked either way. Most manufacturers followed that designation change, but some slipped through the cracks
Clear top (side getter)6CG7/6FQ7 RCA's are also pretty good, but not as good any any of the black plate RCA 6FQ7's. RCA non clear tops (top or bottom getter) are in about the same league as the clear tops.
Sylvanias grey plates are merely o.k., but may apply themselves well to your system. A little more bland presentation, but better than GE grey plates. In both GE and Sylvania, the black plates are superior to the grey plates.
Hope this helps,
One more thing. I was collecting some tubes to send to Cmk for him to try and realized I mis-spoke about the electrostatic shield designation. 6CG7's were specified with the black or shiny electrostatic shield between the plates, 6FQ7's could omit that plate (and usually did).
Sorry for any confusion that may have caused.